This invention relates to a method for detecting a short incident during electrochemical processing and a system therefor.
One industrial process for producing fluorochemical compounds is the electrochemical fluorination process commercialized initially in the 1950s by Minnesota Mining and Manufacturing Company which comprises passing a direct electric current through an electrolyte, (i.e., a mixture of fluorinatable organic starting compound, liquid anhydrous hydrogen fluoride, and perhaps a conductivity additive), to produce the desired fluorinated compound or fluorochemical. This process is commonly referred to as the xe2x80x9cSimons electrochemical fluorination processxe2x80x9d or xe2x80x9cSimons ECFxe2x80x9d. Simons ECF cells typically utilize a monopolar electrode assembly, i.e., electrodes connected in parallel through electrode posts to a source of direct current at a low voltage (e.g., four to eight volts). Simons ECF cells are generally undivided, single-compartment cells, i.e., the cells typically do not contain anode or cathode compartments separated by a membrane or diaphragm. The Simons ECF process is disclosed in U.S. Pat. No. 2,519,983 (Simons) and is also described in some detail by J. Burdon and J. C. Tatlow in Advances in Fluorine Chemistry (M. Stacey, J. C. Tatlow, and A. G. Sharpe, editors) Volume 1, pages 129-37, Buttersworths Scientific Publications, London (1960); by W. V. Childs, L. Christensen, F. W. Klink, and C. F. Kolpin in Organic Electrochemistry (H. Lund and M. M. Baizer, editors), Third Edition, pages 1103-12, Marcel Dekker, Inc., New York (1991); by A. J. Rudge in Industrial Electrochemical Processes (A. T. Kuhn, editor), pages 71-75, Marcel Dekker, Inc., New York (1967); and by F. G. Drakesmith, Topics Curr. Chem., 193, 197, (1997).
Various modifications and/or improvements have been introduced to the Simons ECF process since the 1950s including, but not limited to, those described in U.S. Pat. No. 3,753,976 (Voss et al.); U.S. Pat. No. 3,957,596 (Seto); U.S. Pat. No. 4,203,821 (Cramer et al.); U.S. Pat. No. 4,406,768 (King); Japanese Patent Application No. 2-30785 (Tokuyama Soda KK); SU 1,666,581 (Gribel et al.); U.S. Pat. No. 4,139,447 (Faron et al.); and U.S. Pat. No. 4,950,370 (Tarancon).
U.S. Pat. No. 5,322,597 (Childs et al.) more recently describes the practice in a bipolar flow cell of an electrochemical fluorination process comprising passing by forced convection a liquid mixture comprising anhydrous hydrogen fluoride and fluorinatable organic compound at a temperature and a pressure at which a substantially continuous liquid phase is maintained between the electrodes of a bipolar electrode stack. The bipolar electrode stack comprises a plurality of substantially parallel, spaced-apart electrodes made of an electrically-conductive material, e.g., nickel, which is essentially inert to anhydrous hydrogen fluoride and which, when used as an anode, is active for electrochemical fluorination. The electrodes of the stack are arranged in either a series or a series-parallel electrical configuration. The bipolar electrode stack has an applied voltage difference which produces a direct current which can cause the production of fluorinated organic compound.
Another example of a bipolar flow cell is the Solutia EHD (electrohydrodimerization) cell. See J. Electrochem. Soc.: REVIEWS AND NEWS, D. E. Danly, 131(10), 435C-42C (1984) and Emerging Opportunities for Electroorganic Processes, D. E. Danly, pages 132-36, Marcel Dekker, Inc., New York (1984).
Yet another electrochemical cell is an electrolyzer such as those described by Wullenweber et al. in U.S. Pat. No. 5,174,878. Wullenweber et al. disclose an electrolyzer having bipolar cells arranged in a row and consisting each of two metallic partitions, spring-elastic electrodes bearing on the partitions, and a diaphragm which is disposed between the electrodes and spaced from the electrodes by spacers.
Although electrochemical processing is an effective method to manufacture chemicals, for example fluorochemicals, on occasion the electrochemical cell(s) may experience a xe2x80x9cshort incidentxe2x80x9d. A xe2x80x9cshort incidentxe2x80x9d is defined herein as an event such as for example, a run-away chemical reaction, unstable two-phase flow, or plate flexing which ultimately may lead to a connection of comparatively low resistance made between points on a circuit between which the resistance is normally much greater than zero. The ultimate cause of serious damage from a short incident is current flow through plate-to-plate contacts and the deposition of sufficient energy at those contacts to melt the electrode (e.g., a metal plate, such as nickel). A short incident can result in considerable damage to the electrochemical cell which often means lost time during manufacturing and expense to repair the cell. Thus, it is desirable to eliminate such short incidents or to the extent possible to reduce damage caused by these short incidents. Detecting a short incident as soon as possible helps to reduce damage to the cell caused by the short incident and in turn helps to minimize lost time during manufacturing. Detecting short incidents before plate-to-plate contact occurs substantially lessens damage.
One method of detecting a short incident is to use a mechanics stethoscope, a device much like the familiar physician""s stethoscope, but which uses a pointed solid rod instead of the familiar bell for making contact with the body being examined. With the Simons ECF cell or with low voltage conventional cells, a mechanics stethoscope can be used to detect vibration in the audible frequencies. However, with a bipolar flow cell where the voltage is possibly higher, perhaps several hundred volts, a mechanics stethoscope should not be used for safety reasons. Further, the ability to detect a short incident using a mechanics stethoscope varies with the user""s ability and thus may at times not be reliable, and it is not practical.
Another method of detecting a short incident is to use a large plastic-handled screwdriver in the same manner as a mechanics stethoscope.
These methods require an operator""s attention, are not reliable, and may take a long time to detect the onset of a short incident. Damage to the cell pack and lost production time is often considerable before a short incident is confirmed and a cell is taken out of service.
Another method of detecting a short incident is to monitor current and voltage responses. If the current is set, the voltage fluctuates. However, the xe2x80x9cnoisexe2x80x9d in the voltage can be relatively large as compared to the fluctuation in the voltage due to an individual short incident (for example a 2 to 3 volt drop out of 500 to 600 volts). Thus, it is often difficult, if not impossible to detect a problem until several plates in a cell are affected. It may be possible to monitor voltages between individual plates or set of plates. However, this may be difficult to engineer, may lead to leaks in the system, and involves more data collection, storage, etc.
Thus, the need exists for a method of detecting a short incident in an electrochemical cell which is safe for lower voltage electrochemical cells and for higher voltage electrochemical cells; is reliable; and preferably is inexpensive.
The present invention provides a method for detecting a short incident in an electrochemical cell. The method of the present invention advantageously detects a short incident soon after initiation and thus limits damage, is safe for both low and high voltage electrochemical cells, and is reliable. Further, the method of the present invention is inexpensive to install and to operate.
The method of the present invention utilizes a means for detecting vibration (e.g., an accelerometer) of sufficient sensitivity to detect movement of an externally-located piece of the cell system.
The method of the present invention is a method of detecting a short incident in an electrochemical cell comprising the steps of:
(a) providing an electrochemical cell system having a cell pack and a coupling device, said coupling device being in connection with said cell pack, wherein during electrochemical cell system operation, said coupling device vibrates;
(b) providing a means for detecting vibration of said coupling device, said means for detecting vibration being rigidly affixed to said coupling device;
(c) providing a means for analyzing and monitoring vibration of said coupling device, said means for analyzing and monitoring being connected to said means for detecting vibration;
(d) operating said electrochemical cell system causing said coupling device to vibrate;
(e) detecting said vibration using said means for detecting vibration;
(f) allowing said means for detecting vibration to send a signal to said means for analyzing and monitoring vibration;
(g) continuously measuring, analyzing, and monitoring said vibration to establish a normal baseline amplitude envelope;
(h) operating said electrochemical cell system until said means for analyzing and monitoring vibration measures an amplitude excursion outside of said normal baseline amplitude envelope;
(i) sending a signal to an indicator; and
(j) discontinuing operation of said electrochemical cell.
In one aspect of the present invention the coupling device is a bus bar, the means for detecting vibration is an accelerometer, and the means for analyzing and monitoring vibration is an analog-to-digital converter to a computer.
Another aspect of the present invention is a system for detecting a short incident in an electrochemical cell where the system comprises:
(a) an electrochemical cell system comprising an electrochemical cell pack;
(b) a coupling device being in connection with the cell pack;
(c) a means for detecting vibration rigidly affixed to said coupling device;
(d) a means for analyzing and monitoring vibration of said coupling device connected to said means for detecting vibration; and
(e) an indicator;
wherein said coupling device vibrates during electrochemical cell system operation, said means for detecting vibration detects said vibration and transmits a signal to said means for analyzing and monitoring vibration to establish a normal baseline amplitude envelope, and wherein when a short incident occurs the signal amplitude exceeds the normal baseline amplitude envelope which triggers said indicator and said electrochemical cell operation is discontinued.
The electrochemical cell of the present invention can be a conventional Simons ECF cell, a bipolar flow cell, an electrolyzer such as that described in U.S. Pat. No. 5,174,878, or other electrolytic cells and devices.